PROJECT SUMMARY Skin-penetrating nematodes, including the human threadworm Strongyloides stercoralis, are parasitic roundworms that infect nearly one billion people worldwide. They are intestinal parasites that cause chronic gastrointestinal distress as well as stunted growth and long-term cognitive impairment in children. Infections with S. stercoralis can be fatal for immunosuppressed individuals. Despite the health burden caused by parasitic nematodes, many aspects of their basic biology remain poorly understood. In particular, remarkably little is known about how parasitic nematodes respond to host-emitted sensory cues. In this proposal, we will investigate how parasitic nematodes respond to heat. Heat is emitted by all mammals and is a robust sensory cue for many parasites, including nematodes. We will investigate how heat triggers two different steps of the parasite-host interaction: host seeking, the process whereby the soil-dwelling infective larvae actively search for hosts to infect; and activation, the process whereby the infective larvae exit their developmentally arrested state and resume growth after entering the host. First, we will conduct a quantitative analysis of the temperature-driven host- seeking behaviors of S. stercoralis (Aim 1). Second, we will elucidate the molecular and cellular mechanisms that underlie these behaviors (Aim 2). Third, we will investigate the molecular and cellular mechanisms by which heat triggers activation (Aim 3). These experiments will leverage our recent development of a CRISPR-Cas9 system for targeted mutagenesis in S. stercoralis, allowing us to identify genes required for host seeking and activation. Together, our results will provide fundamental insights into how sensory responses shape the interactions of parasitic nematodes with their hosts.